1. Field
The present disclosure pertains to valves and systems for use in chromatography. More particularly, the present disclosure pertains to a heated rotary valve for use in connection with a sample to be separated in a column for heating the sample to temperature within the ports of the valve. The heated rotary valve permits introduction of a wide boiling range compounds into a gas chromatograph with improved precision of area and retention time.
2. Description of the Related Art
Gas chromatography (GC) is generally performed on a sample using a column positioned within and heated by an oven or other heating device, wherein the sample is heated before introduction to the column. The separated sample is then introduced to a detector for identification of compounds. Chromotagraphy ovens may be operated at elevated temperatures in the range of 40′ C to 400′ C. Because the column is typically a coil of thin tube, such as of metal or fused silica, with an internal polymer coating, the column rapidly reaches the ambient temperature within the oven or applied by an external heating element, which permits movement of the sample through the column. Problematically, the sample needs to be heated to the elevated temperature of the column. This is sometimes accomplished using a heated injection port where the sample size is sufficiently small, such that the injection can be accomplished using a syringe which peirces a septum—a thick, rubber disk. Heated injection ports are sufficiently hot that the sample boils and is carried into the column as a gas by helium or another carrier.
Historically, liquid injection for gas chromatography has included liquid injection by a syringe, whether split on column, but which, while simple, suffered from issues of septum lifetime and an automation system more complex that associated with a valve. The historic alternative was injection by valve, whether a one zone system such as standard liquid sample valves) or two zone injection systems, in which a sampe is moved from a cool zone, such as a sample supply, to a hot zone, such as the column. While more easily controlled and repeatable, these valve systems suffered the difficulty of ensuring the sample was at the temperature of the hot zone when reached.
Additionally, because of the large internal volumes of the valves known in the art, not only was heating unfeasible, but recycling of samples through columns repeatedly for high separation was largely only theoretical as those valves, typically using releatively large fitting adapters ( 1/16 inch or 1/32 inch, for example) introduced peak broadening in each switch, as those fittings were incompatible with small bore columns. Alternatives have included use of Dean's switching and other pressure differential methods using external solenoid valves to control the carrier gas direction. None have been highly effective for column switching.
It would be desirable to incorporate a sample which may be introduced according to a mechanically controlled system and which would be at temperature for processing through the chromatography system, but otherwise would not transfer heat or suffer from heat soak.
Additionally, in chromatography systems, it is typical that the sample may be flowed through a valve during times when no analysis is desired or ongoing. Providing a valve at an elevated temperature during those off-analyzing periods in contact with a flowing sample could be detrimental to the sample, as it could result is repeated vaporization of the sample prior to analysis.
It would therefore be desirable to provide a valve for communication with a sample source, where the valve would permit sample to flow to the column while simultaneously heating the sample to temperature for during the analysis period, but which would otherwise not introduce heat into the surrounding system or to the sample.
It would therefore be desirable to provide a valve having an internal volume of only a few nanoliters which could be used for column switching at an elevated temperature without undesirable cooling or unnecessary additional heating equipment.